The correct answer is B. plunged.
Plunged means to move or fall suddenly and violently into something. In this case, the city was plunged into darkness, meaning that it was suddenly and violently deprived of light.
The other options are not as accurate. Spread means to extend or become more widespread. Merged means to combine or join together. Deep means extending far down from the surface.
In this case, the city was not extended or combined with darkness, nor was it extended far down from the surface. Instead, it was suddenly and violently deprived of light. Therefore, the correct answer is B. plunged.